1. Field of the Invention
The present invention relates to methods and products for the synthesis of oligosaccharide or polysaccharide structures, on glycoproteins, glycolipids, or as free molecules.
2. Discussion of the Background
Carbohydrates are an important class of biological compounds which are remarkable for their structural diversity. This diversity is not random but rather consists of specific sets of oligosaccharide structures that exhibit precise tissue-specific and developmental expression patterns. In cells carbohydrates function as structural components where they regulate viscosity, store energy, or are key components of cell surfaces. Numerous site specific intercellular interactions involve cell surface carbohydrates. For example, union of sperm and egg as well as the implantation of fertilized egg are both mediated by cell surface carbohydrates. Likewise, a number of proteins that function as cell adhesion molecules, including GMP-140, Endothelial Leukocyte Adhesion Molecule-1 (ELAM-1), and lymphocyte adhesion molecules like Mel-14, exhibit structural features that mimic lectins, and are now known to bind specific cell surface carbohydrate structures (Feizi, Trends Biochem. Sci. (1991) 16:84-86). Glycosylated proteins as tumor-associated antigens are now being used to identify the presence of numerous carcinomas. Even isolated oligosaccharides have been found to exhibit biological activity on their own.
Specific galactose oligosaccharides are known to inhibit the agglutination of uropathogenic caliform bacteria with red blood cells (U.S. Pat. No. 4,521,592). Other oligosaccharides have been shown to possess potent antithrombic activity by increasing the levels of plasminogen activator (U.S. Pat. No. 4,801,583). This same biological activity has been used, by binding oligosaccharides, in conjunction with an amino glycoprotein, in medical instruments to provide medical surfaces which have anticoagulation effects (U.S. Pat. No. 4,810,784). Still other oligosaccharides have found utility as gram positive antibiotics and disinfectants (U.S. Pat. Nos. 4,851,338 and 4,665,060). Further, oligosaccharides have been used as bacteria receptor sites in the diagnosis and identification of specific bacteria (U.S. Pat. Nos. 4,657,849 and 4,762,824).
It is also well recognized that oligosaccharides have an influence on the protein or lipid to which they are conjugated (Rademacher et al, Ann. Rev. Biochem., (1988) 57:785). Specific oligosaccharides have been shown to influence proteins' stability, rate of in vivo clearance from blood stream, rate of proteolysis, thermal stability and solubility. Changes in the oligosaccharide portion of cell surface carbohydrates have been noted in cells which have become cancerous. Other oligosaccharide changes have been detected during cell differentiation (Toone et al, Tetrahedron Report (1989) 45(17):5365-5422). As such, the significance of oligosaccharides to biological function cannot be understated.
The fundamental role of these materials in molecular biology has made them the object of considerable research, in particulars considerable efforts have been made in organic synthesis to synthesize these materials. Although synthetic approaches to making carbohydrates are quite developed, this technique suffers notable difficulties which relate to the selective protection and deprotection steps required in the available synthetic pathways. These difficulties, combined with difficulties associated with isolating and purifying carbohydrates, and determining their structures, has made it essentially impossible for synthetic organic chemistry to economically produce valuable carbohydrates.
Enzyme-mediated catalytic synthesis would offer dramatic advantages over the classical synthetic organic pathways, producing very high yields of carbohydrates (e.g., oligosaccharides and/or polysaccharides) economically, under mild conditions in aqueous solution, and without generating notable amounts of undesired side products. Such enzymes, which include glycosyltransferases, are however difficult to isolate, especially from eukaryotic, e.g., mammalian sources, because these proteins are only found in low concentrations, and are membrane-bound.
As of 1987, standard molecular cloning approaches which require amino acid sequence information or anti-glycosyltransferase antibodies, had been successfully used to isolate just two eukaryotic, e.g., mammalian glycosyltransferase cDNAs, corresponding to .beta.(1,4)galactosyltransferase (in 1986) and .alpha.(2,6)sialyltransferase (in 1987). In light of the above-noted considerable value of carbohydrates, there is accordingly a strongly felt need for an improved method for isolation of additional glycosyltransferase genes and cDNAs and for their use in carbohydrate synthesis.